Compositions for stable low resistivity resistors

ABSTRACT

In finely divided resistor compositions containing dielectric material and a resistive oxide which is a polynary oxide of the approximate formula A2B2O7, improved compositions additionally comprising finely divided SiO2 or Al2O3 to enhance stability of resistors made therewith. Resistors of such compositions fired on ceramic substrates.

United States Patent [191 Popowich June 11, 1974 COMPOSITIONS FOR STABLE LOW RESISTIVITY RESISTORS [75] Inventor: Michael John Popowich, Niagara Falls, NY.

[73] Assignee: E. I. du Pont de Nemours and Company, Wilmington, Del.

22 Filed: July 25, 1972 21 Appl. No.: 275,037

Related US. Application Data [63] Continuation-in-part of Ser. No. 246,976, April 24-,

I972, abandoned.

[52] US. Cl 252/520, 252/518, 117/229 [51] Int. Cl. HOlb U015 [58] Field of Search 252/518, 520, 521;

[56] References Cited UNITED STATES PATENTS 3,553,109 l/l97l Hoffman ll7/227 3,630,970 l2/l97l Nelson 252/5l8 Primary Examiner-John D. Welsh 57 7 ABSTRACT 13 Claims, No Drawings COMPOSITIONS FOR STABLE LOW RESISTIVITY RESISTORS CROSS-REFERENCE TO RELATED APPLICATION This is a continuation-in-part of my copending application Ser. No. 246,976, filed April 24, l972 now abandoned.

BACKGROUND OF THE INVENTION This invention relates to resistor compositions, and, more particularly, to improved resistor compositions exhibiting reduced resistivity drift at room temperature.

Various metal oxides have been found to be useful in formulating compositions for printing on dielectric substrates and firing to form thick film resistors in electronic circuits. Such oxides are referred to herein as resistive metal oxides and include polynary metal oxides of the formula M is at least one metal selected from the group consisting of yttrium, indium, cadmium, lead and the rare earth metals of atomic number 577l, inclusive;

M is at least one metal selected from the group consisting of platinum, titanium, tin, chromium, rhodium, iridium, ruthenium, zirconium, antimony and germanium;

x is a number in the range 2; and z is a number in the range 01 being at least equal to about /2 when M is a divalent metal.

Compositions of these resistive metal oxides and dielectric material, and optionally noble metal and/or binary oxides, are described, for example, in Schubert US. Pat. No. 3,560,410; Hoffman [15. Pat. No. 3,553,109; Popowich US. Pat. No. 3,630,969; and Bouchard U.S. Ser. No. 77,309, filed Oct. l, 1970, now allowed, now US. Pat. No. 3,681,262.

A problem has been observed in resistors of such oxides; specifically, fired resistors made from low resistivity compositions, e.g., 100 ohms per square and lower (for l-mil thick resistors), tend to change in absolute resistance value (drift) upon storage under room temperature no-load conditions. The degree of resistance change is strongly dependent upon the sheet resistivity of the resistor composition used and the size of the fired resistor. Under worst case conditions, e.g., sheet resistivities of ohms per square or lower and resistor lengths of 0.040 inch or shorter, resistance changes after 1,000 hours at room temperature may be as high as percent.

Due to the problem of room temperature resistance drift, the low resistivity oxide compositions have found commercial usage when usedin conjunction with a glass-ceramic protective overcoat (e.g, Du Pont Encapsulant Composition 8185), which eliminates the room temperature drift problem. This approach of providing an overcoat, of course, involves an additional screen printing and firing step, which adds cost to the circuit.

Brady US. Pat. No. 3,655,440 relates to resistor compositions said to produce resistors stable at high voltages (column 1, line 40). Resistive oxides shown are those of PdO, RuO and IrO Alumina and barium ferrite are said to control the growth of the crystals of PdO, RuO and IrO which is said to produce such resistors stable at high voltages. However, there is a need for resistor compositions of polynary metal oxides capable of producing fired resistors which combine low resistivity (about ohms per square or less) with reduced low temperature drift, without requiring encapsulation.

SUMMARY OF THE INVENTION In finely divided compositions for printing resistor materials on dielectric substrates, said compositions comprising resistive metal oxides and dielectric material, wherein the resistive oxide is a polynary oxide of the formula (M Bi )(M')O wherein M is at least one metal selected from the group consisting of yttrium, indium, cadmium, lead and the rare earth metals of atomic number 57-71, inclusive;

M is at least one metal selected from the group consisting of platinum, titanium, tin, chromium, rhodium, iridium, ruthenium, zirconium, antimony and germanium;

x is a number in the range 02; and z is a number in the range O-l, being at least equal to about X/2 when M is a divalent metal,

this invention provides improved compositions additionally comprising a free additive oxide which is A1 0 as corundum, SiO or a mixture thereof, in amounts varying from an amount effective'to reduce room temperature drift in fired resistors of such compositions up to about 25 percent, based on the weight of said resistive metal oxide, said additive oxide having an average particle size in the range 0. l5 microns.

Also provided are dispersions of such improved compositions in inert vehicle and resistors of such compositions obtained by printing and firing the improved compositions on a dielectric substrate.

DETAILED DESCRIPTION Compositions of the resistive metal oxides described above and dielectric material (e.g., glass) and optional components (e.g., noble metals and binary metal oxides such as Co O etc.) have been found useful in printing (usually by screening or mask printing in an inert liquid vehicle) on dielectric substrates and firing to produce thick film resistors. This invention is an improvement invention regarding such compositions and fired resistors thereof. Resistive oxide synthesis, printing and firing techniques are those known in the art from, inter alia, the patents mentioned above. Selection and proportions of materials (resistive oxide, dielectric material, vehicle) will depend upon the application technique used and resistor properties desired; the present invention relates to an additive in known systerns.

Known compositions exhibit significant room temperature drift in resistivity, the degree of which is dependent upon several variables, including sheet resistivity of the resistor composition, length of the fired resistor, type of termination material, and thickness of termination material.

The present invention provides improved compositions and resistors thereof, additionally comprising certain additive oxides. The additive oxide is A1 0 SiO or mixtures thereof. The crystalline fonn of the A1 0 used is corundum. The SiO may be either fused silica,

or crystalline silica such as quartz, tridymite or cristobalite. Fused silica is preferred among the forms of silica.

The amount of additive oxide is based upon the weight of the resistive oxide in the composition, and varies from an amount effective to reduce room tem perature drift up to about 25 percent, expressed as weight percent of the weight of resistive oxide. Dependent upon the particular system employed, generally at least 2 percent additive is present to produce a significant reduction in drift.

The average particle size of the additive oxide is quite important and is in the range 0.1- microns, with substantially all particles less than microns in diameter. The preferred additive is A1 0 as corundum. Especially useful are two commercially available grades of alumina. One is a very fine grade having an average particle size in the range 0.3 1 micron and a fairly broad particle size distribution (Alcoa XA16 alumina). The other grade is coarser (average particle size in the range 3-4 microns) with a very narrow particle size distributi0n(A1coa XA141).

ln firing the improved resistor compositions of the present invention, it is important that the additive oxide remain substantially undissolved and appear as discrete particles in the fired resistor. Although not intended to be limiting, generally peak firing temperatures in the range 750 to 860C, for 4-15 minutes at peak, are used.

EXAMPLES AND COMPARATIVE SHOWINGS applying and firing Pd/Ag (V2) end terminations and.

then screen printing (165 mesh screen) the resistor composition, on prefired dense alumina (96 percent) substrates. The firing in each run consisted of a 45- lynary oxide Bi Ru O the frit was a lead bor'osilicate frit (63 percent PbO, 26 percent SiO 10 percent B 0 and 1 percent A1 0 No additive oxide was employed. In Showings A and B, 51.8 g. bismuth ruthenate, 25.0 g. frit and 1.0 g. Pt were dispersed in sufficient vehicle to make 100 g.; in Showings C and D, 43.7 g. Bi Ru O (bismuth ruthenate), 31.0 g. frit and 1.0 g. Pt were dispersed in sufficient vehicle to make 100 g. The solids/- vehicle ratio was 3/1.

Table 1 presents typical resistance changes after 1,000 hours for 10 ohms per square and 100 ohms per square compositions. Under worst case conditions (10 ohms per square composition; 0.040 inch long resistors), changes as high as 20 percent were observed.

These showings illustrate the need for improved resistor compositions.

EXAMPLES 14; SHOWlNGS E AND F The polynary oxide and frit of Comparative Showings A-B were used and A1 0 was substituted for some of the frit in Examples 1-4, as noted in Table 11. The A1 0 was Alcoa XA16, average particle size about 0.3-1.0 micron. The solid/vehicle ratio was 3/1. The resultant 1-mil thick resistors were 40 mils long; performance data are shown in Table 11.

In Table 11 low-R resistors without A1 0 addition are seen to drift markedly (Showing E) at various times, whereas in Examples 3 and 4, at 6 percent A1 0 based on total solids in dispersion 10.7. percent A1 0 based on resistive oxide), drift is minimized to a striking degree.

1n Showing F, alumina alone without frit is seen to be unacceptable due to very high resistivity and rate of drift.

TABLE II Compositions Wt. '70 A1 0 Initial (Based on Total Based on R Example/Showing lnorganics) Polynary Oxide (ohm/sq.) 7! AR 7r Frit 7r A110. 13.5 hr. 40.5 hr. 137 hr. 8040 hr. E 18.0 0 0 5.5 +0.59 +1.80 +4.84 +1S.5 1 17.1 0.9 1.6 5.4 +0.51 +0.72 +1.44 I +9.31 2 16.2 1.8 3.2 5.4 +0.04 +0.80 +1.21 +9.14 3 12.0 6.0 10.7 8.2 0.01 0.02 0.01 +0.33 4 12.0 6.0 10.7 9.4 +0.01 N.D." 0.02 +0.38 F 0 18.0 32.0 1600 +74.2 N.D.* N.D.* +1361 ND. means not determined.

minute cycle with 6-7 minutes at peak temperature EXAMPLE 5 (760C), and heating and cooling rates of about 40C./minute.

The resistivity of the fired resistors was then determined after firing and after the number of hours of standing under no-load conditions, as indicated in each example.

COMPARATIVE SHOWINGS A-D In these showings the resistive oxide used was the po- In this example silica (fused quartz) was used as the additive oxide. A composition was prepared from 49.5 parts Bi Ru O 2.5 parts Pt, 18.5 parts of the lead borosilicate frit of Showings A-D, 0.5 parts of a cadmium borosilicate frit (78 percent CdO, 9.0 percent SiO 4.0 percent A1 0 and 9.0 percent B 0 6 parts fused quartz (-325 mesh) and 23 percent vehicle. There was 12.1 percent 510 based on Bi Ru O Resistors prepared as above had an initial resistivity of l516 ohms/square; percent AR after 21 hours was +0.23 percent, and after 42 hours H).3O percent.

I claim:

1. Finely divided compositions for printing resistor materials on dielectric substrates, said compositions comprising resistive metal oxides and dielectric material, wherein the resistive oxide is a polynary oxide of the formula I 2-I)( '2) 7-Z wherein M is at least one metal selected from the group consisting of yttrium, indium, cadmium, lead and the rare earth metals of atomic number 57-71, inclusive;

M is at least one metal selected from the group consisting of platinum, titanium, tin, chromium, rhodium, iridium, ruthenium, zirconium, antimony and germanium; x is a number in the range 0-2; and z is a number in the range Ol, being at least equal to about /2 when M is a divalent metal, said compositions additionally comprising a free additive oxide which is Al O as corundum, SiO or a mixture thereof, the additive oxide being in an amount effective to reduce room temperature resistivity drift in fired resistors of such compositions, the amount of the free additive oxide being up to about 25 percent by weight, based on the weight of said resistive metal oxide, said additive oxide having an average particle size in the range 0.1-5 microns.

2. Compositions according to claim 1 wherein the additive oxide is A1 0 as corundum.

3. Compositions according to claim 1 wherein the additive oxide is SiO 4. Compositions of claim 1 dispersed in an inert liquid vehicle.

5. Compositions of claim 2 dispersed in an inert liquid vehicle.

6. Compositions of claim 3 dispersed in an inert liquid vehicle.

7. Compositions according to claim 2 wherein the average particle size of the A1 0 is in the range 0.3-4 microns.

' 8. Improved film resistors on dielectric substrates, useful in electronic circuits, which resistors exhibit reduced room temperature resistivity drift, said resistors comprising the resistive metal oxides of claim 1, dielec tric material, and, as an additive oxide, A1 0 as corundum, SiO or mixtures thereof, in amounts ranging from 2 up to 25 percent, based on the weight of the resistive metal oxide, the additive oxide having an average particle size in the range 0. l5 microns.

9. Improved resistors according to claim 8 wherein the additive oxide is A1 0 as corundum.

10. Improved resistors according to claim 8 wherein the additive oxide is SiO 11. Improved film resistors on dielectric substrates, useful in electronic circuits, said resistors comprising the resistive metal oxide of claim 1, dielectric material and, as an additive oxide, A1 0 as corundum, SiO or mixtures thereof, the additive metal oxide being in an amount effective to reduce room temperature resistivity drift, the amount of the additive oxide being up to about 25 percent by weight, based on the weight of said resistive metal oxides, said additive oxide having an average particle size in the range 0. l5 microns.

12. Improved resistors according to claim 11 wherein I the additive oxide is A1 0 as corundum.

13. Improved resistors according to claim 11 wherein the additive oxide is SiO 

2. Compositions according to claim 1 wherein the additive oxide is Al2O3 as corundum.
 3. Compositions according to claim 1 wherein the additive oxide is SiO2.
 4. Compositions of claim 1 dispersed in an inert liquid vehicle.
 5. Compositions of claim 2 dispersed in an inert liquid vehicle.
 6. Compositions of claim 3 dispersed in an inert liquid vehicle.
 7. Compositions according to claim 2 wherein the average particle size of the Al2O3 is in the range 0.3-4 microns.
 8. Improved film resistors on dielectric substrates, useful in electronic circuits, which resistors exhibit reduced room temperature resistivity drift, said resistors comprising the resistive metal oxides of claim 1, dielectric material, and, as an additive oxide, Al2O3 as corundum, SiO2 or mixtures thereof, in amounts ranging from 2 up to 25 percent, based on the weight of the resistive metal oxide, the additive oxide having an average particle size in the range 0.1-5 microns.
 9. Improved resistors according to claim 8 wherein the additive oxide is Al2O3 as corundum.
 10. Improved resistors according to claim 8 wherein the additive oxide is SiO2.
 11. Improved film resistors on dielectric substrates, useful in electronic circuits, said resistors comprising the resistive metal oxide of claim 1, dielectric material and, as an additive oxide, Al2O3 as corundum, SiO2 or mixtures thereof, the additive metal oxide being in an amount effective to reduce room temperature resistivity drift, the amount of the additive oxide being up to about 25 percent by weight, based on the weight of said resistive metal oxides, said additive oxide having an average particle size in the range 0.1-5 microns.
 12. Improved resistors according to claim 11 wherein the additive oxide is Al2O3 as corundum.
 13. Improved resistors according to claim 11 wherein the additive oxide is SiO2. 